Apparatus and method for separating air by cryogenic distillation

ABSTRACT

The invention relates to equipment for separating air that includes a first average pressure distillation column, a second low-pressure distillation column, a third low-pressure distillation column, a first condenser-vaporizer, a second condenser-vaporizer arranged in the tank of the second column, a pipe for supplying air to the first column, a pipe for supplying a nitrogen-enriched liquid from the head of the first column to the head of the third column, at least one pipe for supplying the liquid from the tank of the third column to the first condenser-vaporizer, a pipe for supplying a liquid of the tank from the first column to the third column, a pipe for supplying the liquid from the first condenser-vaporizer to the second column, a pipe for supplying gas from the head of the second column to the tank of the third column, wherein the first, second, and third columns are arranged side by side.

The present invention relates to an apparatus and a method for separating air by cryogenic distillation.

It is known, for a conventional apparatus for separating air (of the “double column” type), to install:

-   -   the low pressure column above the average pressure column with         heat exchange carried out by the vaporizer;     -   the low pressure column and the average pressure column         side-by-side, with liquid return pumps (either liquid oxygen         from the low pressure column to the vaporizer-condenser or         liquid nitrogen from the vaporizer-condenser to the average         pressure column) and pipes for transporting gas (return of         gaseous oxygen from the vaporizer-condenser to the low pressure         column and the transmission of gaseous nitrogen from the average         pressure column to the vaporizer-condenser).

It is known that the first arrangement makes it possible to obtain improved energy performance as it saves loss of pressure in the gas return pipes (predominant effect) and energy from the liquid return pumps.

It is known, for an apparatus for separating air having two vaporizer-condensers in the low pressure column, to install:

-   -   in arrangement 1: the low pressure column (with its two         vaporizers) above the average pressure column (FIGS. 1 and 4,         FR-A-2724011).     -   in arrangement 2: the bottom section of the low pressure column         on the floor surface and the top section of the low pressure         column above the average pressure column so as to carry out heat         exchange directly via the overhead vaporizer of the average         pressure column (FIGS. 2 and 3 of FR-A-2724011).     -   in arrangement 3: the low pressure column (with its two         vaporizers) to the side of the average pressure column (U.S.         Pat. No. 6,134,915, EP-A-0195065).

It is known that arrangements 1 and 2 permit an equivalent energy performance to be obtained. In both cases, there is loss of pressure in the gas pipes and no liquid return pump).

There is no document which proposes to position the upper part of a low pressure column on the floor surface, or even an upper part of the low pressure column which does not contain a condenser-reboiler.

Surprisingly, for very low-energy methods using a double column with at least two vaporizers in the low pressure column, the conclusion is drawn that the best performing arrangement in terms of energy consumption is to install:

-   -   the average pressure column on the floor surface,     -   the upper section of the low pressure column on the floor         surface,     -   the lower section of the low pressure column on the floor         surface.

The main reason is that the difference in pressure between the average pressure column and the low pressure column is typically less than 2 bar absolute pressure, which necessitates the installation of a pump for raising liquid in the case where sections of the low pressure column are installed above the average pressure column.

According to a subject of the invention, an apparatus is provided for separating air comprising a first distillation column capable of operating at an average pressure, a second distillation column capable of operating at a low pressure, a third distillation column capable of operating at a low pressure or at the low pressure, a first condenser-vaporizer, a second condenser-vaporizer arranged in the bottom of the second column, a pipe for supplying air at least to the first column, a pipe for supplying a nitrogen-enriched liquid from the head of the first column to the head of the third column, at least one pipe for supplying liquid from the bottom of the third column to the first condenser-vaporizer, a pipe for supplying a liquid from the bottom of the first column to the third column, a pipe for supplying liquid from the first condenser-vaporizer to the second column, a pipe for supplying overhead gas from the second column to the bottom of the third column, the first, second and third columns being arranged side-by-side, possibly all being arranged on a floor surface.

Preferably, the second column contains at least a third condenser-vaporizer placed above the second condenser-vaporizer.

An apparatus element, for example a liquid oxygen vaporizer, capable of operating at a cryogenic temperature, may be arranged below one of the columns and wherein the two other columns and the element are placed directly on the floor surface.

Pressurization means may be connected to the bottom of the second column and to the liquid oxygen vaporizer.

The first condenser-vaporizer may be above the first column and connected to the head of the first column to permit reboiling by a gas from the first column.

Preferably, the apparatus comprises just one pump connected to two of the columns.

The pump may be connected to the two pipes for supplying liquid from the bottom of the second column to the first condenser-reboiler.

A gas pipe may connect the first condenser to the bottom of the third column.

The head of the second column and/or the third column may be at a lower level than the first condenser-reboiler and preferably lower than the head of the first column.

The packing densities in the first column may be lower than the packing densities of the second column and/or of the third column.

Preferably, the third column does not contain a condenser-reboiler.

The second column may also contain a third condenser-reboiler in addition to means for exchanging heat and material arranged above the third condenser-reboiler.

The pipe for supplying overhead gas from the second column to the bottom of the third column may be connected to a pipe for gas vaporized in the first condenser-reboiler.

The apparatus element is preferably a liquid oxygen vaporizer, arranged below the third column.

The difference in pressure between the operating pressure of the first column and the pressure of at least one of the second and third columns is preferably less than 3 bar, preferably 2.5 bar, preferably 2 bar.

The difference in pressure between the operating pressure of the top of the second column and the bottom of the third column may be less than 1 bar, preferably less than 0.5 bar, preferably less than 0.2 bar.

The invention will be disclosed in more detail by referring to the figures which show apparatuses for separating air according to the invention.

In FIG. 1, the apparatus comprises a first column 1 operating at an average pressure, a second column 2 operating at a low pressure and a third column 3 operating at a low pressure which is slightly less than that of the second column 2, the low pressure columns preferably operating at a difference in pressure of less than 500 mbar (and preferably 200 mbar) measured between the bottom of the column 3 and the top of the column 2. The difference in pressure between the average pressure column 1 and the low pressure column 2 or 3, measured in the middle of each column is less than 3 bar, preferably 2.5 bar, preferably 2 bar.

The three columns 1, 2, 3 are placed on the floor surface 27, but the third column 3 is raised as a product vaporizer 11 is placed below said column. It is naturally possible to place the vaporizer 11 elsewhere and to place the column 3 directly on the floor surface 27. The floor surface 27 consists of a concrete base or other planar surface.

A compressed air flow 33, which is purified and cooled is passed into the bottom of the first column 1 where it is separated, forming an oxygen-enriched flow 25 and a nitrogen-enriched flow. The oxygen-enriched flow 25 is passed to a lower level of the third column 3. The nitrogen-enriched flow condenses in the overhead condenser 9 of the first column. Said condenser 9 is cooled by a flow of liquid 23 pumped from the bottom of the third column 3 and pressurized by a pump 13 which is also placed on the floor surface 27. The vaporized liquid 17 from the condenser 9 is mixed with an overhead gas 15 of the second column 2 to form a gaseous flow 19 which supplies the bottom of the third column 3. The head of the column of the second column 2 is supplied by a liquid 21 from the condenser 9.

The second column 2 contains one or two, or more, reboilers, including a bottom reboiler 5 and possibly an intermediate reboiler 7. Said reboilers may be heated up by any suitable means, including an air flow, whether cold compressed or not, or a nitrogen flow, whether cold compressed or not. A flow of gaseous or liquid oxygen 29 is withdrawn at a low level of the second column 2. Said flow of liquid may be pressurized by a pump (not shown) and passed to the vaporizer 11 to form a pressurized gaseous flow.

A liquid nitrogen flow 35 is expanded in a valve and passed to the head of the third column 3 and a nitrogen-enriched gaseous flow 31 is withdrawn at the head of the third column 3. Preferably, the head of the third column 3 is lower than the head of the first column 1, thus facilitating this transfer by gravitational flow.

The difference between FIG. 2 and FIG. 1 is that FIG. 1 shows a condenser 9 which is a bath vaporizer, whilst FIG. 2 shows a film vaporizer 9. In this case, the flow 21 withdrawn from the condenser is a two-phase flow and passed to the head of the second column as in FIG. 1. No flow 17 is produced.

FIGS. 3 and 4 are variants of FIGS. 1 and 2, respectively showing the case where the vaporizer 11 is absent or placed elsewhere other than below one of the columns. In this case, the three columns rest directly on the floor surface 27.

FIGS. 5 and 6 respectively show variants of FIGS. 1 and 2, where the second column contains just one reboiler, the bottom reboiler 5. In this case, the second column is naturally shorter. The bottom reboiler 5 may be heated by nitrogen or air, which is possibly compressed, or possibly cold compressed.

According to the invention, it is understood that at most one, or even two, pumps are necessary. The first pump is the pump 13 used for raising liquid from the bottom of the third column to the overhead condenser of the first column. The second pump, used in the case where the oxygen is required in pressurized form, is used to pressurize the liquid oxygen. For raising the other liquids, such as the liquid 25, the difference in pressure between the columns should suffice to entrain the liquid.

The first column 1 contains relatively low packing densities (for example a density of 250 to 500 m²/m³) and the second and/or third column contains relatively high packing densities (600 to 1000 m²/m³) in order to achieve the highest possible first column 1, and the lowest possible second and/or third column 2, 3 so that the flow may take place by gravitational force between the vaporizer located above the first column to the second and/or to the third column. The passage of the liquids in the pipes 21, 35 is facilitated in this manner. 

1-14. (canceled)
 15. An apparatus for separating air comprising; a) a first distillation column, comprising a head and a bottom, capable of operating at a first pressure, b) a second distillation column, comprising a head and a bottom, capable of operating at a second pressure, c) a third distillation column, comprising a head and a bottom, capable of operating at a third or at the second pressure, d) a first condenser-vaporizer, e) a second condenser-vaporizer arranged in the bottom of the second column, f) a pipe for supplying air at least to the first column, g) a pipe for supplying a nitrogen-enriched liquid from the head of the first column to the head of the third column, h) at least one pip for supplying liquid from the bottom of the third column to the first condenser-vaporizer, i) a pipe for supplying a liquid from the bottom of the first column to the third column, j) a pipe for supplying liquid from the first condenser-vaporizer to the second column, and k) a pipe for supplying overhead gas from the second column to the bottom of the third column, the first, second and third columns being arranged side-by-side
 16. The apparatus of claim 15, wherein the first, second and third columns are all arranged on a floor surface.
 17. The apparatus of claim 15, wherein a liquid oxygen vaporizer capable of operating at a cryogenic temperature, is arranged below one of the columns and wherein the two other columns and the liquid oxygen vaporizer are placed directly on the floor surface.
 18. The apparatus of claim 17, comprising a means for pressurization connected to the bottom of the second column and to the liquid oxygen vaporizer.
 19. The apparatus of claim 15, wherein the first condenser-vaporizer is above the first column and is connected to the head of the first column to permit reboiling by a gas from the first column.
 20. The apparatus of claim 15, further comprising a single pump connected to two of the columns.
 21. The apparatus of claim 20, wherein the pump is connected to the two pipes for supplying liquid from the bottom of the second column to the first condenser-reboiler.
 22. The apparatus of claim 15, further comprising a gas pipe connecting the first condenser-reboiler to the bottom of the third column.
 23. The apparatus of claim 15, wherein the head of the second column and/or the third column is at a lower level than the first condenser-reboiler and the packing densities in the first column may be lower than the packing densities of the second column and/or of the third column.
 24. The apparatus of claim 23, wherein the head of the second column and/or the third column is lower than the head of the first column.
 25. The apparatus of claim 15, wherein the third column does not contain a condenser-reboiler.
 26. The apparatus of claim 15, wherein the second column also contains a third condenser-reboiler in addition to means for exchanging heat and material arranged above the third condenser-reboiler.
 27. The apparatus of claim 15, wherein the pipe for supplying overhead gas from the second column to the bottom of the third column is connected to a pipe for gas vaporized in the first condenser-reboiler.
 28. The apparatus of claim 15, wherein the liquid oxygen vaporizer is arranged below the third column.
 29. A method for separating air using an apparatus as claimed in claim 15, wherein the difference in pressure between the operating pressure of the first column and the pressure of at least one of the second and third columns is less than 3 bar.
 30. The method of claim 29, wherein the difference in pressure between the operating pressure of the first column and the pressure of at least one of the second and third columns is 2.5 bar.
 31. The method of claim 30, wherein the difference in pressure between the operating pressure of the first column and the pressure of at least one of the second and third columns is 2 bar
 32. The method for separating air using an apparatus as claimed in claim 15, wherein the difference in pressure between the operating pressure of the top of the second column and the bottom of the third column is less than 1 bar.
 33. The method of separating air of claim 32, wherein the difference in pressure between the operating pressure of the top of the second column and the bottom of the third column is less than 0.5 bar.
 34. The method of separating air of claim 33, wherein the difference in pressure between the operating pressure of the top of the second column and the bottom of the third column is less than 0.2 bar. 